1. Field of the Invention
The present invention relates to a metal halide lamp and a method for manufacturing the same, and more particularly to a metal halide discharge lamp, achieving a considerable improvement in visibility and therefore used as an alternative to an incandescent bulb, such as a halogen bulb, which has been used as a light source in conventional vehicle head lamps, and a method for manufacturing the same.
2. Background Art
FIG. 5 shows an example configuration of a conventional metal halide lamp 90 of this type. As shown in the figure, the metal halide lamp 90 has a discharge chamber (a burner) 92 formed of a quartz glass bulb 91. A pair of tungsten electrodes 93 are provided inside the discharge chamber 92 with their respective discharge ends 93a facing each other. Tungsten coils 94 are fixed around the electrodes 93, and molybdenum foils 95 are welded to the ends of the electrodes 93 which are opposite to the discharge ends 93a. In addition, lead-in wires 96 for supplying current from outside to the electrodes 93 are connected to the molybdenum foils 95.
FIG. 6 shows the main parts of the metal halide lamp 90. When the electrode 93 is directly contacted to the quartz glass bulb 91, cracks in the quartz glass bulb 91 is caused and consequently leaks occurs due to the thermal expansion of the electrode 93 resulting from the increase in temperature when the lamp is lit. Therefore, the tungsten coil 94 having a greater inner diameter than the outer diameter of the electrode 93 is fixed around the electrode 93 in order to prohibit direct contact between the electrode 93 and the quartz glass bulb 91, thereby preventing such cracks.
However, in the conventional metal halide lamp 90 constructed as above, a gap D is created between the electrode 93 and the quartz glass bulb 91 by fixing the tungsten coil 94 around the electrode 93. As shown in FIG. 6, this gap D reaches as far as the end of the electrode 93 to which the molybdenum foil 95 has been provided.
As a result, there is a problem that metal halide and mercury sealed within the discharge chamber 92 escape along the gap D to reach the molybdenum foil 95, thereby causing peeling off the molybdenum foil 95 from the quartz glass bulb 91. Such peeling-off may cause leaks in the discharge chamber 92, shortening the useful life of the metal halide lamp 90.
Furthermore, when metal halide and mercury enter into the gap D as described above, the amount of metal halide and mercury sealed within the discharge chamber 92 may consequently be decreased, resulting in a reduced amount of light when the lamp is lit. In the case of a head lamp, a reduction in the amount of light to 70% of the initial amount of light is regarded as necessitating the replacement of the head lamp. Therefore, the useful life of the lamp may be reduced even when no leaks actually occur in the discharge chamber 92 as described above.